High hydrostatic pressure is now widely recognized as an important controllable environmental parameter both in industrial manufacturing technologies and in laboratory investigations. The proliferating variety of process control applications combined with increasing data transmission rates has resulted in the need for improved accuracy and reliability in pressure measurement. Other needs include compatibility with data transmission systems, multiplexing and demultiplexing of large sensor arrays, and both distributed and "smart" sensing. Although a wide choice of physical properties would seem to be available for use in sensing devices, most sensors currently in use depend ultimately on resistance, capacitance or frequency.
Sensors employing optical fibers have significantly greater sensitivity, are very small and can be configured in arbitrary shapes so they can penetrate into often inaccessible locations. They can sense nearly any physical perturbation and at the same time have inherent immunity to electromagnetic interference (EMI), which is an important feature in noisy industrial environments. Finally, fiber-optic sensors are intrinsically safe so they are well suited for electrically dangerous, hazardous or explosive environments including high-pressure and hightemperature conditions. By definition they are readily compatible with fiber-optic telemetry and optical data transmission systems. Possible applications include not only high-pressure laboratory measurement but also oil field and mining instrumentation and many general process-control and industrial manufacturing technologies.
One sensor has been reported so far in the publication entitled "HIGH PRESSURE FIBER-OPTIC SENSOR WITH SIDE-HOLE FIBER" by K. JANSEN et al., pages 56 to 60 of Proceedings of SPIE, vol. 798, Fiber Optic Sensors II (1987), with measurement characteristics provided for pressure up to about 70 MPa. The device is based on uniaxially induced elasto-optic birefringence occurring in side-hole optical fibers under hydrostatic stress conditions.
With conventional high-pressure sensors, a wide range of physical properties of different materials can be used as a basis for high-pressure sensing. Specific requirements for a practical device, however, such as sensitivity, thermal stability and lack of hysteresis severely limit this range to a few preferred phenomena. For measuring pressures up to about 1 GPa, sensors based on resistance, capacitance or frequency changes under pressure are most often used.
Resistance gauges using alloy materials such as manganin, gold-chromium or other 3d or 4f alloys are classical, with pressure coefficients ranging from 1.multidot.10.sup.-5 MPa.sup.-1 to 3.multidot.10.sup.-5 MPa.sup.-1. Pure or doped semiconductors or semiconductor devices such as light emitting diodes could also be included in this group, with pressure coefficients ranging from 8.multidot.10.sup.-5 MPa.sup.-1 to 4.4.multidot.10.sup.-4 MPa.sup.-1. Capacitance sensors are known to have good long-range stability with moderate pressure coefficients such as 5.9.multidot.10.sup.-5 MPa.sup.-1 for calcite. Ultrasonic gauges, based on resonance frequency shift under pressure in quartz or other crystals, were reported to have low pressure coefficients of about 4.multidot.10.sup.-5 MPa.sup.`1 and good accuracy, but because they are usually large and require techniques of ultrasonic interfereometry, they are more appropriate for laboratory experiments than for practical industrial applications.
Many fiber-optic low-pressure sensors were developed primarily as modifications of displacement or stress transducers using multi-mode technology. They are generally based on one of two principal ideas: the first uses fibers as waveguides to and from a bulk sensing element and the element itself changes its transmission properties under pressure; the second is based on using the fiber itself as a sensing element. Recently two phase-sensitive transducers were reported: in both cases the unilateral force depending on external pressure induces birefringence in low-birefringent fiber, as described in the publication entitled "HIGH RESOLUTION PHOTOELASTIC PRESSURE SENSOR USING LOW BIREFRINGENCE FIBER" by A. BERTHOLDS et al. Appl. Opt. vol. 25, pp 340-3 (1986) or in side-hole fiber, as described by K. Jansen. Induced birefringence, defined as a difference in propagation constants .beta..sub.x and .beta..sub.y for two principal polarization axes, increases with pressure and generates the output signal.
To date only the above mentioned paper of K. JANSEN has dealt with fiber-optic sensing of high pressure, i.e. pressure of at least 60-70 MPa. As a measured parameter, the authors used pressure-induced birefringence occurring in a side-hole single-mode fiber with a fixed length of 10 cm. The fiber was placed directly inside a pressure capillary and sealed at both ends with an epoxy resin. The output of this sensor, resulting from pressure-induced birefringence in a single-mode non-birefringent fiber having two circular cavities parallel to the core, was found to be a periodic function of pressure up to about 70 MPa. The period of the output signal, however, is only of about 0.8 MPa, and is therefore not well suited for measurement of higher pressures since calibration procedure of such a sensor would be unnecessarily difficult.
Also known in the art is a U.S. Pat. No. 4,740,078 by Rene DAENDLIKER et al., issued on Apr. 26th, 1988. In this patent, there is described method and apparatus for the measurement of force by the use of stressinduced birefringence in a single-mode optical fiber. In this case, the apparatus comprises, among other things, force-responsive means for laterally compressing a curved optical fiber portion, the force-responsive means including at least two parallel spaced pressure plates having planar adjacent surfaces on opposite sides of, and in engagement with, said optical fiber curved portion.
The apparatus of DAENDLIKER et al. is aimed at a goal different from the one of the present application. The apparatus of DAENDLIKER et al. measures the pressure along a specific axis, on the contrary, the present application describes an apparatus for measuring the ambient pressure in an environment. The apparatus of DAENDLIKER et al. is also different from the apparatus described in the present application in that it requires two parallel spaced pressure plates for compressing the curved optical fiber portion by which a pressure exerted along a specific axis, can be detected.
It is an object of the pressure invention to overcome certain drawbacks and limitations of high pressure sensors known in the art.
It is another object of the present invention to provide a birefringent optical fiber device for measuring high pressure, using a highly birefringent optical fiber, which device is economical to construct and easy to operate.
It is another object of the present invention to provide a birefringent optical fiber device adaptable to any pressure vessel or apparatus using a wide variety of high pressure medium such as gas or liquid.
It is another object of the present invention to provide a birefringent optical fiber device which is reuseable and which is capable of being rapidly assembled and disassembled in a body of a chamber or a connector.
It is another object of the present invention to provide a birefringent optical fiber device for measuring high pressure in explosive or harsh environments without using any electrical signals and eliminating in the same time the risk of electrical short-circuits in the system.
It is another object of the present invention to provide a birefringent optical fiber device for measuring any required range of pressures up to 200 MPa.